1. Field of the Invention
The disclosed subject matter is generally directed to Advanced Metering Infrastructure (AMI), via which data is communicated between a utility, such as an electric power company, and meters that are located at the premises of the customers of the utility. The exemplary embodiments are particularly aimed at efficient and economical communications between the meters and servers at the back office or other central facility of the utility.
2. Description of Related Art
Power line carrier (PLC) or power line communications, also known as Power line Digital Subscriber Line (PDSL), mains communication, power line telecom (PLT), or power line networking (PLN), is a system for carrying data on a conductor also used for electric power transmission. Broadband over Power Lines (BPL) is an example of a system that uses PLC by sending and receiving information-bearing signals over power lines to provide access to the Internet.
Typically, electrical power is transmitted over high voltage transmission lines, distributed over medium voltage lines, and used inside buildings at lower voltages. Power line communications can be applied at each stage. Most PLC technologies limit themselves to one set of wires, e.g., premises wiring, but some can cross between two stages, e.g., both the distribution network and premises wiring.
Known power line communications systems operate by impressing a modulated carrier signal on the wiring system. Different types of power line communications use different frequency bands, depending on the signal transmission characteristics of the power wiring being used. Since the power wiring system was originally intended for transmission of AC power at a nominal frequency of, e.g., 50 or 60 Hz, the power line circuits have only a limited ability to carry higher frequencies. This propagation problem is a limiting factor for each type of power line communications.
Data rates over a power line communication system vary widely. Low frequency (about 100-200 kHz) carriers impressed on high-voltage transmission lines may carry one or two analog voice circuits, or telemetry and control circuits with an equivalent data rate of a few hundred bits per second. However, these circuits may be many miles long. Higher data rates generally imply shorter ranges. A local area network operating at millions of bits per second may only cover one floor of an office building, but eliminates installation of dedicated network cabling.
Power line communications can also be used to interconnect home computers, peripherals or other networked consumer peripherals. Proprietary specifications for power line home networking have been developed by a number of different companies within the framework of the HomePlug Powerline Alliance, the Universal Powerline Association and the HD-PLC Alliance.
Broadband over power lines (BPL), also known as power-line Internet or powerband, is the use of PLC technology to provide broadband Internet access through ordinary power lines. A computer (or any other device) only needs to plug a BPL “modem” into any outlet in an equipped building to have high-speed Internet access. International Broadband Electric Communications, or IBEC, and other companies currently offer BPL service to several electric cooperatives.
BPL may offer benefits over regular cable or DSL connections: the extensive infrastructure already available enables people in remote locations to access the Internet with relatively little equipment investment by the utility. In addition, such ubiquitous availability would make it much easier for other electronics, such as televisions or sound systems, to be connected.
However, variations in the physical characteristics of the electricity network and the current lack of IEEE standards mean that provisioning of the service is far from being a standard, repeatable process. In addition, the amount of bandwidth a BPL system can provide, compared to cable and wireless, is in question.
Known PLC systems have a number of issues. The primary one is that power lines are inherently a very noisy environment. Every time a device turns on or off, it introduces a pop or click into the line. Energy-saving devices often introduce noisy harmonics into the line. The system must be designed to deal with these natural signaling disruptions and work around them.
Power distribution uses step-down transformers to reduce the voltage for use by customers. However, BPL signals cannot readily pass through transformers, as their high inductance makes them act as low-pass filters, blocking high-frequency signals. Thus, repeaters must be attached to the transformers. Broadband over power lines has developed faster in Europe than in the United States due to a historical difference in power system design philosophies.
In the U.S., it is common for a small transformer hung from a utility pole to service a single house or a small number of houses. In Europe, it is more common for a somewhat larger transformer to service 10 to 100 houses. For delivering power to customers, this difference in design makes little difference for power distribution. However, delivering BPL over the power grid in a typical U.S. city requires an order of magnitude more repeaters than in a comparable European city. On the other hand, since bandwidth to the transformer is limited, this can increase the speed at which each household can connect, due to fewer people sharing the same line. One possible solution is to use BPL as the backhaul for wireless communications, for instance by mounting Wi-Fi access points or cell phone base stations on utility poles, thus allowing end-users within a certain range to connect with equipment they already have. BPL may also be used as a backhaul for WiMAX networks.
Another issue is signal strength and operating frequency. Known systems have operated at frequencies of 10 to 30 MHz, which have been used for many decades by amateur radio operators, as well as international shortwave broadcasters and a variety of communications systems (military, aeronautical, etc.). Power lines are unshielded and will act as antennas for the signals they carry, and have the potential to interfere with shortwave radio communications.
Known technologies that communicate over a shared power line bus have been concerned with the speed and reliability of the communications. Reliability may be addressed through the use of error correction codes and other algorithms designed to mitigate the unique noise environment of the power lines. Frequency adaptive and/or speed variable modulator/demodulators (modems) have been employed to increase reliability as well. For example, modern BPL systems may use OFDM modulation, which allows them to mitigate interference with radio services by removing specific frequencies that are used.
Applications of power line communications vary widely, as would be expected of such a widely available medium. One natural application of narrow band power line communication is the control and telemetry of electrical equipment, such as meters, switches, heaters and domestic appliances. A number of known developments have considered such applications from a systems point of view, such as “Demand Side Management.” In this implementation, domestic appliances would intelligently coordinate their use of resources, for example, limiting peak loads.
Control and telemetry applications include both “utility side” applications, which involve equipment belonging to the utility company, i.e., between the supply transformer substation up to the domestic meter, and “consumer-side” applications, which involve equipment in the consumer's premises. Possible utility-side applications include automatic meter reading (AMR), dynamic tariff control, load management, load profile recording, credit control, pre-payment, remote connection, fraud detection and network management, and could be extended to include gas and water.
Utility companies also use special coupling capacitors to connect medium-frequency radio transmitters to the power-frequency AC conductors. Frequencies used are in the range of 24 to 500 kHz, with transmitter power levels up to hundreds of watts. These signals may be impressed on one conductor, on two conductors or on all three conductors of a high-voltage AC transmission line. Several PLC channels may be coupled onto one high voltage line. Filtering devices are applied at substations to prevent the carrier frequency current from being bypassed through the station apparatus and to ensure that distant faults do not affect the isolated segments of the PLC system. These circuits are used for control of switchgear, and for protection of transmission lines. For example, a protection relay can use a PLC channel to trip a line if a fault is detected between its two terminals, but to leave the line in operation if the fault is elsewhere on the system.
One aspect of the use of this type of shared media has not been previously addressed, namely, the simultaneous communication by multiple nodes or devices on the shared bus. This is because, to date, the shared use of power lines has been accomplished by time-division access to the media. This approach is more applicable when all the communicating units are members of the same installation, particularly when they are all being controlled by a common controlling node. In the case of autonomous transmissions by the various units, however, such an approach becomes impractical.